(i) Field of the Invention
The present invention relates to the prevention of scorching before crosslinking of a thermoplastic and/or elastomeric composition with peroxides or azo compounds.
(ii) Description of Related Art
Premature crosslinking (scorching) during the preparatory phase is a major difficulty in the use of peroxides and azo compounds in crosslinking (curing) applications of elastomeric and/or thermoplastic materials. The preparatory phase consists in general in blending the constituents and optionally extruding them at temperatures that are often high. The operating conditions of this preparatory phase quite often lead to decomposition of the peroxide or azo initiator, thus inducing the crosslinking reaction with formation of gel particles in the bulk of the blend. The presence of these gel particles leads to imperfections (inhomogeneity or surface roughness) of the final product. Excessive scorching reduces the plastic properties of the material, such that the said material can no longer be converted, leading to loss of the entire batch. In addition, excessive scorching may lead to the total stoppage of the extrusion operation.
Several solutions have been proposed to overcome this drawback. Thus, it has been proposed to use an initiator with a half-life of 10 hours at high temperature. The drawbacks of this approach are the low production efficiency due to a long curing time and the high energy costs.
It has also been proposed to incorporate certain additives in order to reduce the scorch tendency. Thus, the use of organic hydroperoxides as scorch inhibitors for polyethylene-based compositions crosslinked with a peroxide was described in British patent GB 1,535,039. The use of vinyl monomers was the subject of patent U.S. Pat. No. 3,954,907. The use of nitrites was described in patent U.S. Pat. No. 3,202,648. Patent U.S. Pat. No. 3,335,124 describes the use of aromatic amines, phenolic compounds, mercaptothiazole compounds, sulphides, hydroquinones and dialkyl dithiocarbamate compounds.
Very recently, the use of 2,2,6,6-tetramethylpiperidyloxy (TEMPO) and 4-hydroxy-2,2,6,6-tetramethylpiperidyloxy (4-hydroxy TEMPO) was the subject of a Japanese patent application JP 11-49865.
However, the use of the additives of the art cited above to increase the scorch-resistance time has a harmful effect on the curing time and/or on the final crosslinking density. It leads, therefore, to a reduction in the production efficiency and/or properties of the final product.